In telecommunications, modulation is one of most important processes, which varies a periodic waveform, where a message can be conveyed by usually changing some parameters of a sine wave such as amplitude, phase, and frequency. All of these parameters can be modified in accordance with a low frequency information signal to obtain the modulated signal. There are many ways to modulate a signal: amplitude modulation (AM), frequency modulation (FM), phase modulation (PM), and pulse modulation (PM). Among the modulations, PM is the most common modulation scheme and is able to be further classified into pulse amplitude modulation (PAM), pulse code modulation (PCM), pulse frequency modulation (PFM), pulse position modulation (PPM), and pulse width modulation (PWM).
In order to increase the number of modulation dimensions, a well-known modulation scheme so called multidimensional signal modulation can be used to construct signal waveforms corresponding to higher-domain. The higher-domain may rely on either the time domain or the frequency domain or both.
In recent telecommunication standards such as in 3GPP, 3GPP2, and 802.16, the most common uses of modulation is quadrature amplitude modulation (QAM) whereby an adaptive modulation and channel coding (AMC) scheme can be easily employed in order to aim in improving the overall system performance in accordance with the experienced channel condition by the receiver in time, frequency, or spatial domain.
However, all modulation schemes like QAM convey data by changing some aspect of a carrier signal, or the carrier wave, (usually a sinusoid) in response to a data signal. In the most application, the modulation mechanism is unvaried; for example, mechanism between PPM and QAM is not changed or not combined. Even with AMC scheme, it only varies the modulation order, such as from quadrature phase shift keying (QPSK) to 16QAM or to 64QAM, and vice versa.
Although many efforts were put to optimize the adaptation with existing modulation methods so far, the conventional approaches that improve individual methods had a problem that they limit the overall system performance.